Multiple times programmable (MTP) memory circuits are a particular kind of semiconductor non-volatile memory (NVM), providing re-programmable storage of data, with long retention, without need for refresh or application of power.
MTP memory circuits typically comprise a plurality of cells, each cell typically storing one bit of binary information. MTP cells storing more than one bit of information, sometimes referred to as “multi-level” MTPs, are known. Further detailed discussion of these is omitted, because concepts and features of the present invention may be applied to multi-level MTPs by persons of ordinary skill in the art based on this disclosure.
MTP cells are often fabricated on the same substrate as other circuitry such as, for example, analog filters, to provide local storage of, for example, amplifier gain and phase adjustment values, and to store identification data such as, for example, radio frequency identification device (RFID) parameters.
MTPs are designed and fabricated to meet a plurality of design goals. Example goals include data retention, convenience in programming, storage capacity, density, power consumption, and manufacturing cost. Some of these goals interrelate with one another. For example, density relates to storage capacity. Some of these goals may be measured by various different metrics. For example, manufacturing cost may include a count of the number of added processing steps, meaning steps not common with any step performed in fabricating any non-MTP circuitry. Manufacturing cost may include a decrease in chip yield arising from, for example, process faults within the MTP cells. Manufacturing cost may be based, in part, on the commonality (or lack of commonality) of processing steps forming the MTP cells with processing steps forming other circuit components.
As known with MTPs, for certain ones of these design goals, optimization with respect to one goal may be detrimental with respect to one or more of the other goals.
For example, conventional electrically erasable programmable memory (EEPROM), have functionality that overlaps with MTP but generally provide larger capacity. However, EEPROM typically requires numerous extra processing steps. Further, semiconductor system forming, on a common substrate, CMOS devices and bipolar devices, are being employed in increasing numbers, over an increasing range of applications. BiCMOS processing, though, has particular issues that often magnify the cost of added fabrication steps. Conventional EEPROM processing, in addition to requiring many steps, requires steps that are typically not included in conventional BiCMOS processing.